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Radioactive isotope, maybe from Fukushima, detected, but ...

August 15, 2011 | 3:17
pm

This post has been corrected. See the note at the bottom for details.

Very small amounts of a radioactive isotope of sulfur, believed to have traveled across the Pacific Ocean from the Fukushima nuclear plant in Japan, have been detected in La Jolla, Calif., by UC San Diego scientists.

But there's no need to worry: The amounts are nowhere near enough to cause health problems, researchers said.

Senior author Mark Thiemens and his team keep tabs on levels of sulfur-35 as part of their climate research. Readings collected shortly after the March 11 tsunami in Japan indicated that there were 1,500 atoms of sulfur-35 per cubic meter of air in La Jolla, a significant increase over normal levels.

The UCSD team interpreted the bump as the result of a reaction that would have occurred when plant workers used seawater to cool overheating reactors at Fukushima. Neutrons from the reactor core would have reacted with chlorine in ocean water to create radioactive sulfur, Thiemens said.

"The levels we observed are in no way harmful in California," Thiemens said.

The group reported the measurements Monday in the journal Proceedings of the National Academy of Sciences.

Thiemens and his colleagues use highly sensitive instruments to detect the minuscule amounts of radioactive sulfur that circulate naturally in the atmosphere. That they detected a bump in levels of radioactive sulfur was "not surprising," said Kai Vetter, who teaches radiation detection at UC Berkeley. Vetter's lab has been tracking incoming radiation from Japan and has reported upticks too -- though again, nothing that would pose a danger to people in the U.S.

But Vetter and other nuclear engineers questioned elements of the research, which used the readings taken in La Jolla to extrapolate the amount of neutron leakage from the Fukushima plant. Elmer Lewis of Northwestern University and Michael Golay of MIT were unconvinced that the radiation in question even originated at the nuclear plant.

Edward Morse, of UC Berkeley, said that the traces of radioactive sulfur probably originated at Fukushima, but he took issue with the team's final calculations.

"They're not nuclear engineers," Morse said. "They were a little out of their depth."

For the Record, 8:45 p.m. Aug. 15: An earlier version of this post said that readings indicated there were 1,500 atoms of sulfur-35 per square meter of air in La Jolla. It should have said per cubic meter.

Photo: Tsunami waves approach tanks of heavy oil for the Unit 5 reactor of the Fukushima Daiichi nuclear complex in Okuma, Fukushima Prefecture, northeastern Japan, in this March 11 photo released by Tokyo Electric Power Co. The waves unleashed by a magnitude-9 earthquake destroyed backup generators for several reactors' cooling systems, and the nuclear cores in three reactors melted. Credit: AP Photo/Tokyo Electric Power Co.